A buck converter is a step-down DC to DC converter. It is a switched-mode power supply that typically uses two switches, an inductor and a capacitor.
A multiphase buck converter is a DC to DC converter that provides multiple low voltage and high current outputs, which are required by processors, memory banks, and ASICs, for example. The multiphase buck converter has interleaved phases, and a multiphase controller provides control signals to the buck converter to control those phases.
The simplest way to reduce the voltage of a DC supply is to use a linear regulator. However, linear regulators waste energy by dissipating excess power as heat. Buck converters, on the other hand, can be more efficient. This efficiency and reduced heat make them useful for tasks such as converting the main voltage in a system down to a lower voltage required by a processor included in the system.
One of the biggest challenges in design of power supplies for systems including processors is handling transient response requirements. For example, a processor can demand in excess of 100 A of current in a matter of nanoseconds. A power supply has to maintain output voltage within a very tight tolerance. Such transient response requirements put a huge burden on the power supply.
The most common way that multiphase buck converters operate is in voltage mode fixed frequency operation. A proportional-integral-derivative (PID) compensator controls the converter. See, for example, U.S. Pat. No. 7,245,512 to Leung, incorporated herein by reference.